1. Field of the Invention
This invention relates to a constant force device, a movement and a mechanical timepiece.
2. Description of the Related Art
In a mechanical timepiece, if a rotation torque transmitted to an escape wheel & pinion of an escapement from a barrel drum varies in response to unwinding of a main spring in the barrel drum, an oscillation angle of a balance with hairspring is changed and a rate of a timepiece is changed. Therefore, in order to suppress the variation in the rotation torque transmitted to the escape wheel & pinion, a constant force device is proposed in which a constant force spring (pre-tensioning spiral spring) is arranged between the barrel drum and the escapement.
For example, as the constant force device, those which include a stop wheel & pinion having a stop pinion (stop wheel pinion), an escape wheel & pinion having an escape pinion (escape wheel shaft), a tensioning ring attached to a tensioning ring pinion, a constant force spring disposed between the tensioning ring and the escape wheel & pinion, and a cam attached to the escape pinion are proposed. The constant force spring supplies a rotation force to the escape wheel & pinion so that the escape wheel & pinion is rotated with respect to the tensioning ring (for example, refer to Japanese Patent No. 4105941 (Patent Reference 1)).
According to this configuration, whereas rotation of the escape wheel & pinion is stopped or resumed by a pallet of a first anchor, rotation of the stop wheel & pinion is stopped or resumed by a pallet of a second anchor. The second anchor is caused to perform an oscillating movement by a fork-shaped portion engaging with the cam. Then, if the rotation of the stop wheel & pinion is resumed, the tensioning ring is rotated. In this manner, the constant force spring is wound up on a regular basis. Accordingly, it is possible to suppress the variation in the rotation torque transmitted to the escape wheel & pinion.
Incidentally, in the above-described related art, the rotation of the stop wheel & pinion is stopped or resumed by causing the second anchor to perform the oscillating movement using the cam and the fork-shaped portion. Consequently, there is a problem of an increasing loss of power for stopping or resuming the rotation of the stop wheel & pinion.